Insulation resistance test in vacuum 4

[Powerykh]

Hello guys, i have a question on insulation resistance (IR) test.
When we conducted IR test with 5kV DC on a 132kV GIS busduct which is already degassing and in vacuum stage (means SF6 gas removed, and is under vacuuming close to 1 mbar), the insulation resistance value for the busduct L1 phase (or L2 phase, or L3 phase) to ground was very low (about 20Megaohm). But when the busduct filled with SF6, the IR value become 40Gigaohm.
I am a bit confusing, as i thought vacuum also a good insulation ? as we have vacuum circuit breaker which can withstand high voltage?

 

[btrueblood]

Google the term "Paschen's Law".

 

[Powerykh]

Hi btrueblood, thank you very much!

 

[prc]

In power transformers it is a cardinal rule that never apply voltage under vacuum as it may result in insulation breakdown in windings.

 

[Zogzog]

Your vacuum level is not low enough to be an effective insulator.

 

[Powerykh]

Thank you prc and Zogzog